Austenitic chromium-nickel stainless steels are enjoying an increasing wide-spread industrial application as engineering alloys because of their resistance to corrosion and desirable mechanical properties. Unfortunately, these steels are not significantly hardenable by or responsive to heat treatment; phase transformation is suppressed by the nickel constituent in these steels and austenite (the gamma form of iron) is substantially retained on cooling from the gamma region. The mechanical properties of these austenitic steels cannot be controlled or varied by the usual types of heat treatment, such as quenching and tempering. Changes in mechanical properties, such as strength, are brought about only by expensive, time-consuming cold working (rolling) and annealing, resulting in the so-called wrought austenitic stainless steel. (See "Forming of Austenitic Chromium-Nickel Stainless Steels" 2nd Ed. (1954), published by The International Nickel Co., Inc., New York, N.Y.).
In accordance with this invention, desirable changes in austenitic chromium-nickel stainless steels are brought about by a certain novel powder metallurgy technique, resulting in a sintered or unwrought alloy having increased strength and other desirable mechanical properties.
Powder metallurgy broadly is not a new type of metallurgical process but it is receiving increasing application in the manufacture of metallic articles, extending as it does the design limits of liquid metallurgy. An excellent description of this metallurgical process is found in "Review of the Powder Metallurgy Process," July, 1966, published by the U.S. Army Production Equipment Agency, Manufacturing Technology Division, Rock Island Arsenal, Illinois.
Powder metallurgy has been used to make metal articles approaching the physical properties, such as density and strength, of cast or wrought alloys of similar composition. In fact, powder metallurgy has been applied to stainless steel (see "Progress in Powder Metallurgy", Vol. 16, pp 120-129, 1960, Capital City Press, Montpelier, Va.). Although useful stainless steel articles have been made by the powder metallurgy technique, generally high compacting pressures and prolonged sintering at elevated temperatures have been found necessary in order to produce high density articles. Generally stainless steel articles commercially produced by powder metallurgy procedures have densities of 80-90 percent of theoretical density and interconnected porosity. These densities are not as high as desired and result in mechanical properties that are not as good as those of annealed wrought articles of similar composition, and the interconnected porosity increases their susceptibility to corrosion. Further, in order to obtain desired strengths, it generally has been necessary to coin or mechanically work the sintered stainless steel articles.
By further way of background of the prior art, mention should be made of U.S. Pat. No. 2,593,943 (Wainer) which discloses molding mixtures of metal powders with a heat-fugitive binder, the metal powders employed therein including powdered molybdenum, nickel, cobalt, and other metals, as well as mixtures of both a metal and an alloy. However, there is no teaching in this patent of sintering a powdered metal mixture of austenitic chromium-nickel stainless steel and exclusively a ferrite stabilizer such as molybdenum, to form an austenitic-ferritic chromium-nickel alloy of increased strength. U.S. Pat. No. 2,792,302 (Mott) discloses making sintered articles from 18-8 stainless steel using 10 to 15 weight % of a binder which can contain a relatively small amount of molybdenum disulfide as a die lubricant, which, upon subsequently being reduced during the sintering operation, has an insignificant effect, if any, on the properties of the sintered article. U.S. Pat. No. 3,223,523 (Adler) discloses a powder metallurgy technique in which stainless steel powder (AISI 302) is blended with an aqueous solution of a salt of molybdenum, copper, or nickel, such as ammonium molybdenate, which salt is reduced to form a metallic coating on the stainless steel powder, the amount of metallic coating being sufficient to improve the green strength of the powder compact and apparently less than that which would increase the strength of the resulting sintered article or change the finished properties thereof.